When I am reading accelerometer data (with my settings) from MEGA it returns just zeroes and sometimes '-1'.This is normal, because I set it up with lowest sensitivity (16g) and accelerometer just sits on the desk.

But when I run the same code on DUE instead of '-1' I see 65535.

I wonder, does it have anything to do with different int type for DUE?I believe this is all about that line

Probably there is something wrong with signed integer representation. As I understand, when INT is stored in 2 bytes, then '-1' in binary would be just 16 '1'-s, but when this number is interpreted as 4 bytes, it is equal to 65535. I am not quite good in understanding details of signed integers representation, so I might be wrong.

No, you've already figured it out. All 16 bits set in a two byte signed integer represents -1; in a 4 byte signed integer, the lower 16 bits set represents 65535 (-1 would be represented by all 32 bits set). The highest bit in a signed integer holds the sign of the number, + or -ve; the rest of the bits hold the actual value. To change the sign of an integer, the rule is to change all the zero bits to ones, and all the one bits to zero. That produces what is called the "one's complement". And then add one to that (using unsigned overflow arithmetic). That produces the "two's complement". If you are curious, you can read why it is represented this way here:

http://en.wikipedia.org/wiki/Two%27s_complement

More practically, for your problem, a quick fix would be to declare the offending 32 bit int as int16_t in the Due code. Then you'll be working with a 16 bit signed int on both platforms.

//This buffer will hold values read from the ADXL345 registers.char values[10];//These variables will be used to hold the x,y and z axis accelerometer values.int x,y,z;

void setup(){ //Initiate an SPI communication instance. SPI.begin(); //Configure the SPI connection for the ADXL345. SPI.setDataMode(SPI_MODE3); //Create a serial connection to display the data on the terminal. Serial.begin(9600);

//Set up the Chip Select pin to be an output from the Arduino. pinMode(CS, OUTPUT); //Before communication starts, the Chip Select pin needs to be set high. digitalWrite(CS, HIGH);

//Put the ADXL345 into +/- 4G range by writing the value 0x01 to the DATA_FORMAT register. writeRegister(DATA_FORMAT, 0x01); //Put the ADXL345 into Measurement Mode by writing 0x08 to the POWER_CTL register. writeRegister(POWER_CTL, 0x08); //Measurement mode }

void loop(){ //Reading 6 bytes of data starting at register DATAX0 will retrieve the x,y and z acceleration values from the ADXL345. //The results of the read operation will get stored to the values[] buffer. readRegister(DATAX0, 6, values);

//The ADXL345 gives 10-bit acceleration values, but they are stored as bytes (8-bits). To get the full value, two bytes must be combined for each axis. //The X value is stored in values[0] and values[1]. x = ((int)values[1]<<8)|(int)values[0]; //The Y value is stored in values[2] and values[3]. y = ((int)values[3]<<8)|(int)values[2]; //The Z value is stored in values[4] and values[5]. z = ((int)values[5]<<8)|(int)values[4];

//This function will write a value to a register on the ADXL345.//Parameters:// char registerAddress - The register to write a value to// char value - The value to be written to the specified register.void writeRegister(char registerAddress, char value){ //Set Chip Select pin low to signal the beginning of an SPI packet. digitalWrite(CS, LOW); //Transfer the register address over SPI. SPI.transfer(registerAddress); //Transfer the desired register value over SPI. SPI.transfer(value); //Set the Chip Select pin high to signal the end of an SPI packet. digitalWrite(CS, HIGH);}

//This function will read a certain number of registers starting from a specified address and store their values in a buffer.//Parameters:// char registerAddress - The register addresse to start the read sequence from.// int numBytes - The number of registers that should be read.// char * values - A pointer to a buffer where the results of the operation should be stored.void readRegister(char registerAddress, int numBytes, char * values){ //Since we're performing a read operation, the most significant bit of the register address should be set. char address = 0x80 | registerAddress; //If we're doing a multi-byte read, bit 6 needs to be set as well. if(numBytes > 1)address = address | 0x40;

//Set the Chip select pin low to start an SPI packet. digitalWrite(CS, LOW); //Transfer the starting register address that needs to be read. SPI.transfer(address); //Continue to read registers until we've read the number specified, storing the results to the input buffer. for(int i=0; i<numBytes; i++){ values[i] = SPI.transfer(0x00); } //Set the Chips Select pin high to end the SPI packet. digitalWrite(CS, HIGH);}

//This buffer will hold values read from the ADXL345 registers.char values[10];//These variables will be used to hold the x,y and z axis accelerometer values.int x,y,z;

void setup(){ //Initiate an SPI communication instance. SPI.begin(); //Configure the SPI connection for the ADXL345. SPI.setDataMode(SPI_MODE3); //Create a serial connection to display the data on the terminal. Serial.begin(9600);

//Set up the Chip Select pin to be an output from the Arduino. pinMode(CS, OUTPUT); //Before communication starts, the Chip Select pin needs to be set high. digitalWrite(CS, HIGH);

//Put the ADXL345 into +/- 4G range by writing the value 0x01 to the DATA_FORMAT register. writeRegister(DATA_FORMAT, 0x01); //Put the ADXL345 into Measurement Mode by writing 0x08 to the POWER_CTL register. writeRegister(POWER_CTL, 0x08); //Measurement mode }

void loop(){ //Reading 6 bytes of data starting at register DATAX0 will retrieve the x,y and z acceleration values from the ADXL345. //The results of the read operation will get stored to the values[] buffer. readRegister(DATAX0, 6, values);

//The ADXL345 gives 10-bit acceleration values, but they are stored as bytes (8-bits). To get the full value, two bytes must be combined for each axis. //The X value is stored in values[0] and values[1]. x = ((int)values[1]<<8)|(int)values[0]; //The Y value is stored in values[2] and values[3]. y = ((int)values[3]<<8)|(int)values[2]; //The Z value is stored in values[4] and values[5]. z = ((int)values[5]<<8)|(int)values[4];

//This function will write a value to a register on the ADXL345.//Parameters:// char registerAddress - The register to write a value to// char value - The value to be written to the specified register.void writeRegister(char registerAddress, char value){ //Set Chip Select pin low to signal the beginning of an SPI packet. digitalWrite(CS, LOW); //Transfer the register address over SPI. SPI.transfer(registerAddress); //Transfer the desired register value over SPI. SPI.transfer(value); //Set the Chip Select pin high to signal the end of an SPI packet. digitalWrite(CS, HIGH);}

//This function will read a certain number of registers starting from a specified address and store their values in a buffer.//Parameters:// char registerAddress - The register addresse to start the read sequence from.// int numBytes - The number of registers that should be read.// char * values - A pointer to a buffer where the results of the operation should be stored.void readRegister(char registerAddress, int numBytes, char * values){ //Since we're performing a read operation, the most significant bit of the register address should be set. char address = 0x80 | registerAddress; //If we're doing a multi-byte read, bit 6 needs to be set as well. if(numBytes > 1)address = address | 0x40;

//Set the Chip select pin low to start an SPI packet. digitalWrite(CS, LOW); //Transfer the starting register address that needs to be read. SPI.transfer(address); //Continue to read registers until we've read the number specified, storing the results to the input buffer. for(int i=0; i<numBytes; i++){ values[i] = SPI.transfer(0x00); } //Set the Chips Select pin high to end the SPI packet. digitalWrite(CS, HIGH);}